The present invention relates to a sensing system for a humidity sensor.
Humidity relates to the amount of water vapor that exists in the air. One measure of humidity is relative humidity which is the amount of water vapor in a sample of air compared to the maximum amount of water vapor the air can hold at any specific temperature. Relative humidity may be defined as the ratio of the partial pressure of water vapor in a gaseous mixture of air and water vapor to the saturated vapor pressure of water at a given temperature. Relative humidity may be expressed as a percentage in the following manner:
  RH  =                    p                  (                                    H              2                        ⁢            O                    )                            p                  (                                    H              2                        ⁢            O                    )                *              ×    100    ⁢    %  where p(H2O) is the partial pressure of water vapor in the gas mixture; p*(H2O) is the saturation vapor pressure of water at the temperature of the gas mixture; and RH is the relative humidity of the gas mixture being considered.
Another measure of humidity is absolute humidity which is the quantity of water in a particular volume of air. The amount of vapor in that volume of air is the absolute humidity of that volume of air. One equation representing absolute humidity (e.g., AH) is the mass of water vapor mw, per cubic meter of air, Va.
  AH  =            m      w              V      a      
Specific humidity is the ratio of water vapor to air (including water vapor and dry air) in a particular volume. Specific humidity ratio is expressed as a ratio of kilograms of water vapor, mw, per kilogram of mixture, mt.
The specific humidity (e.g., SH) ratio can be expressed as:
  SH  =            m      w                      m        u            +              m        v            
Specific humidity is related to a mixing ratio (and vice versa) by:
      SH    =          MR              1        +        MR                  MR    =          SH              1        -        SH            
Other measures of humidity may be used, typically depending on the particular application.
Humidity sensors tend to have limited accuracy in providing a repeatable measurement due to hysteresis within the sensor itself. A humidity sensor with hysteresis may be in any number of states, independent of the inputs to the humidity sensor. More precisely, the hysteresis of a humidity sensor exhibits path-dependence, or rate-independent memory. Accordingly, humidity sensors with hysteresis are problematic to predict the output without knowing the history of the input. Thus, in order to predict the output, the path that the input followed before it reached its present value needs to be known.
Some humidity sensors have limited hysteresis relative to sensor repeatability but tend to be relatively expensive. While generally accurate, such sensors tend to be to expensive for many applications. Other humidity sensors have significant hysteresis relative to sensor repeatability but tend to be relatively inexpensive. While sufficiently inexpensive, such sensors may be too inaccurate for many applications.
What is desired therefore, is a relatively inexpensive sensing system for a humidity sensor that has relatively low dependence on it hysteresis.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.